feat: add interval event loop utilization mode

[mcollina]

Summary

• add an opt-in eventLoopUtilizationMode: 'interval' setting that advances the ELU baseline after every sample
• preserve cumulative-since-registration utilization as the default for backward compatibility
• document and type the option, with a deterministic regression test covering threshold and response behavior

Testing

• npm test
• npm run lint
• git diff --check

Closes #294

[Tony133]

lgtm